1. Field of the Invention
This invention relates generally to telephone switching systems and more particularly to supervisory and control circuits for detecting the operative state of communication lines and effecting control conditions in response thereto.
2. Description of the Prior Art
It is anticipated that in many instances electronic exchanges will be installed in the same building with existing step-by-step exchanges, with the objective of arresting the growth of and eventually phasing out the step-by-step equipment. The combined equipment therefore serves its area as if it were a single switching machine, although it is necessary to provide internal trunks between the two systems so that lines and trunks connected to one system can reach lines and trunks connected to the other. Further, economies are realized if trunk groups to other sites can be treated as a single group, instead of being subdivided into step-by-step and electronic exchange groups. Such a group would then appear on the inlets and/or outlets of one machine; calls to or from lines or trunks on the other therefore require "tandem" connections via interoffice junctors. Two types are required. One type is used for calls advanced from the step-by-step exchange to destinations reached via the electronic exchange; the other is used for calls proceeding in the opposite direction.
It is desirable for reasons of transmission that additional supervisory bridges not be included in connections of this type; i.e., the junctor is preferably "transparent" while in a talking mode, particularly on toll calls. This objective is easily attained when the call is extended from the step-by-step to the electronic exchange: the last selector rank switches through to the electronic exchange incoming circuit. The path is held under program control by ground returned on the C lead by the latter circuit.